Solid polymer electrolyte membrane for use in polymer electrolyte fuel cell shows an ion conductivity only in a state involving water molecules to some extent, and thus its power generation efficiency abruptly decreases when the electrolyte membrane is dried. When the electrolyte membrane is too much wetted with water, on the other hand, the membrane fails to conduct gas diffusion therethrough, resulting in a decrease in the power generation efficiency. Thus, to attain a high and stable power output, the electrolyte membrane must be appropriately humidified. The electrolyte membrane can be humidified by a bubbler humidification system, a water vapor addition system, a humidifying membrane system, etc., among which the humidifying membrane system for humidifying the electrolyte membrane by transporting only water vapor contained in an exhaust gas into a feed gas through a selectively water vapor permeating membrane, that is, a humidifying membrane system is expected to be highly promising because of a possibility to make the humidifier compact in volume and light in weight.
Desirable shape of the humidifying membrane for use in the humidifying membrane system is a hollow fiber membrane having a larger permeation area per unit volume in the case of membrane modules. Very high humidifying capacity is required for humidification of the electrolyte membrane of fuel cells, particularly fuel cells for mobile unit, and thus a porous membrane state is desirable for the hollow fiber membrane because a high permeation rate can be obtained by capillary condensation of water vapor and because the capillary condensation of water vapor can act as a barrier against other gases.
To obtain such a porous membrane, the present applicant formerly proposed a process for producing a polymetaphenylene phthalamide separation membrane by extruding a film-forming solution of polymetaphenylene isophthalamide made by a low-temperature polycondensation process, the solution also containing a water-soluble polymer and an inorganic salt, into a coagulation bath under heating conditions at 70° C. or higher, thereby coagulating the solution (JP-A-2001-286743).
On the other hand, such durability as to ensure a prolonged operation is required for fuel cells, particularly fuel cells for mobile unit, and thus a resistance to moisture and heat without any deterioration, even if subjected to high temperature and high humidity circumstances for a long time, is also required for the humidifying membrane. However, to satisfy the resistance to moisture and heat renders a humidifying membrane low in the humidifying performance, and thus it is difficult to satisfy the two requirements at the same time, i.e. resistance to moisture and heat and humidifying performance.